This invention relates to an improved process for vulcanizing chlorobutyl rubber compositions containing natural and/or synthetic rubber. It also pertains to vulcanizable rubber compositions prepared by this process and the resultant vulcanizates. More particularly, the invention relates to the use of a specific alkyl phenol sulfide, namely, para-tert. butyl phenol disulfide, as a component of the vulcanizing system for chlorobutyl rubber compositions. It relates still further to the resultant vulcanizable compositions and the vulcanizates producible therefrom.
The use of alkyl phenol sulfides as vulcanizing agents for butadiene polymers is well known. For example, Wolf et al in "Alkylphenolsulfides as Vulcanizing Agents", Industrial & Engineering Chemistry 38, 1157 (1946), and Wolf et al U.S. Pat. 2,422,156 (1947) teach the use of alkyl phenol sulfides as curing agents for butadiene polymers such as chloroprene, isoprene and copolymers of 1,3-butadiene with styrene, acrylonitrile and other polymerizable olefins. Extensive investigations at that time led to the recognition that certain para-tert. alkyl phenol disulfides were preferred vulcanizing agents for synthetic butadiene-styrene rubbers (GR-S). This discovery resulted from efforts to find materials which would function as effective tackifiers between the elastomer and the non-rubber components; it was necessary to find such tackifiers in order for the butadiene-styrene rubber to be useful in pneumatic tires.
In U.S. Pat. No. 2,422,156, the patentees point out that a wide variety of possible para-tert. alkyl phenol sulfides can be prepared according to the general reaction scheme: ##SPC1##
R can be a tert. alkyl group from 4 to about 10 carbon atoms. The symbol x can be 1, 2 or a higher number, depending upon how much sulfur was introduced in the reaction. Using sulfur dichloride according to the above indicated scheme, x would be 1; using sulfur monochloride, x would be 2. If a tri - or higher sulfide is desired, the product can be further reacted with elemental sulfur. The repeating unit n can be 0, 1, 2 or a higher digit, and would depend upon the ratio of the reactants used. As employed in the instant specification, the term, "para-tert. alkyl disulfide" refers to compounds in which x is 2. The terminology respecting the various alkyl sulfides, the method of preparation and significant background material may be found in U.S. Pat. No. 2,422,156, the disclosure of which is hereby incorporated by reference.
The Wolf et al article entitled "Alkylphenolsulfides as Vulcanizing Agents", referred to above discusses at length the relationship between tack and vulcanizing ability. It was concluded therein that, with respect to butadiene-styrene copolymers, the preferred alkyl phenol sulfides are disulfides in which, in the above reaction scheme, R has from 4 to 6 atoms and in which the sulfur content of the product ranges from about 20 to about 30 wt.%
The most readily available para-tert. alkyl phenol disulfides were the para-tert. amyl phenol disulfides which could be readily prepared from para-tert. amyl phenol. Two principal commercial products have been introduced, Vultac 2, containing 23% sulfur, and Vultac 3, containing 28% sulfur. The exact structure of these products is not known; it is believed that they are complex mixtures of polysulfides. They are generally referred to in advertising and literature as simply "alkyl phenol disulfides". The terminology is, unfortunately, somewhat misleading in that the products consist principally of compounds having tert. amyl groups and they do not contain any compounds with a tert. butyl group. The Vultac alkyl phenol disulfides appear to have been developed not only as vulcanizing agents (i.e., as a replacement for sulfur), but also as tackifying agents. These commercial alkyl phenol disulfides, which are composed primarily of tert. amyl phenol disulfides, are themselves tacky substances and are therfore quite suitable as tackifying agents in styrene-butadiene rubber compositions. For these and other reasons, they have also found utility in other rubber compositions such as natural rubber and nitrile-butadiene rubbers.
The use of undiluted commercially available alkyl phenol disulfides presents some rather difficult problems in handling. These products have a tendency to coalesce when stored in containers at ambient temperatures. The result is a fused product which presents an extremely difficult problem upon attempts at removal and handling. Efforts to solve this handling problem have led to the development of diluted alkyl phenol disulfide products. For example, the para-tert. amyl phenol disulfide product, Vultac 2, has been fused with about 30% of stearic acid to give a hard wax-like product. This diluted product, sold under the trademark Vultac 4, is considerably easier to remove from a storage container than corresponding undiluted product Vultac 2, but is still not entirely satisfactory. At room temperature, sharp tools are required to remove the product from its container. Preferably, the product is heated to its softening range of about 48.degree.-58.degree.C., thus facilitating removal. This is not, however, a convenient procedure for modern automated rubber manufacturing plants which would prefer to employ a free-flowing powder. Furthermore, since the diluted product contains only 16% sulfur, added processing expenses arise from the fact that more product must be used.
A para-tert. amyl phenol disulfide having 28% sulfur (Vultac 3) is available commercially. This product which is tacky, is also available in a diluted form containing about 30% of Micro-Cel E (a very fine absorptive and porous synthetic calcium silicate) and is sold under the name Vultac 5. It is a chemically and physically stable powder which does not coalesce below 71.degree.C. As with the previously described amyl phenol disulfide product, i.e. Vultac 4, rubber product manufacturers incur additional expense because the sulfur is considerably below the 28% value for the corresponding undiluted form. Furthermore, the product is extremely dusty and its use requires the employment of dust control facilities. One of the more serious objections to the use of this material is the frequent difficulty in obtaining a good dispersion which is a requirement for any rubber additive in order to produce high quality products.
In the 1950's, butyl rubbers were modified notably by addition of small amounts of halogen atoms, notably chlorine and bromine, to the polymer chain. Chlorinated butyl rubber, also known as chlorobutyl rubber, possesses many advantageous properties, such as resistance to environmental attack and low permeability to gases. Futhermore, it is extremely compatible with highly unsaturated elastomers to produce co-vulcanizates using known vulcanizing agents. Further information concerning the properties, characteristics and chemistry of vulcanization of chlorobutyl rubber may be found in Baldwin et al, "Preparation and Properties of Chlorobutyl", Rubber and Plastics Age 42, 500 (1961) the pertinent portions of which are incorporated herein by reference.
Chlorobutyl rubber contains allylic chlorine atoms which are easily removed by chemical reaction and thus afford additional cross-linking sites. The commercially available alkyl phenol disulfides which, as noted above, are composed primarily of tert. amyl phenol disulfides, were found to be good auxiliary cross-linking agents for chlorobutyl rubber. This was not a completely expected phenomenon, because the same commercial alkyl phenol disulfides shows no utilization in brominated butyl polymers. One of the advantages possessed by chlorinated butyl rubbers is that they are themselves tacky and need no additional tackifer. Thus, with chlorobutyl rubber, the use of the commercial para-tert. amyl phenol disulfide compositions serves only the single purpose of vulcanization. This is in contrast to both the early and current styrene-butadiene rubber compositions in which the alkyl phenol disulfides serve as both a vulcanizing agent and as a tackifier. All that is required for chlorobutyl rubber is a vulcanizing agent, but, in order to get this vulcanizing ability, it has been necessary to employ substances which are needlessly inconvenient for use.
It is therefore a principal object of this invention to provide a vulcanizing agent for use in chlorobutyl rubber which would have vulcanizing properties comparable to those of the commercially available para-tert. amyl phenol disulfides but which would be easier to handle.
It is a further object of this invention to provide such vulcanizing agents in easily handled form without the necessity of dilution with inert material.
Further objects will become apparent from the following discussion of the invention.